1. Field of the Invention
The present invention relates to an optical-fiber amplifier, and more particularly to an optical-fiber amplifier that directly amplifies optical signals using an erbium-doped optical fiber (hereinafter referred to as optical fiber) through which pumping light is supplied.
2. Description of the Related Art
Optical-fiber amplifiers constructed from optical fibers introduce pumping light from external energy sources into the optical fibers, and directly amplify signal light by stimulated emission of the erbium in the optical fibers. These optical-fiber amplifiers are used in optical repeaters, optical receivers, and other optical communication equipment.
For optical communication equipment, especially, for optical repeaters for undersea cables, duplicate (or redundant) equipment is used in the pumping light source system because a total failure of the pumping light source system would require enormous amounts of time and money to repair. For bi-directional communications, this requires that a total of four pumping light sources, two for each direction of transmission, be provided in a single optical repeater.
On the other hand, an optical amplifier system that achieves a redundant configuration for the transmission lines of both directions using a total of two pumping light sources, is known. In the known system, light energy from the two pumping light sources is input into an optical coupler which combines the light and then splits it into two outputs that are respectively coupled into optical fibers, one for the forward transmission direction and the other for the backward transmission direction. In the event of a failure and stoppage of one or the other of the pumping light sources, light energy from the remaining pumping light source is supplied to both the forward and backward transmission lines.
The two pumping light sources used have an identical output light wavelength in nominal terms. In such a system, when the wavelength of the light energy from one pumping light source is sufficiently separated from that of the light energy from the other pumping light source, the levels of the two sources are stable. However, if the two wavelengths are very close to each other, or if they become very close to each other due to temperature changes, the wavelengths of the two sources will become equal because of light reflected back into the light sources. If this happens, the two pumping beams interfere, and as a result, the levels of the two output beams from the optical coupler fluctuate in an opposite phase relationship with respect to each other. In this case, if the level of one output beam remains small for some time, there arises the problem that the optical signal that is supposed to be amplified by that beam cannot be amplified.